Electric lamp



April 5, 1938.

ELECTRIC LAMP Filed Nov. 27, 1935 2 Sheets-Sheet 2 Fig. 4.

Fig. 3.

w. H. LE MARECHAL ET AL 2,112,993

Patented Apr. 5, 1938 PATENT OFFICE v ELECTRIC LAMP William Henry Le Marchal and John Norman Aldington, Ashton-on-Ribble, England, assignors to Siemens Electric Lamps and Supplies Limited, London, England, a British company Application November 2'7, 1935, Serial No. 51,758 In Great Britain December 8, 1934 5 Claims.

This invention relates to electric lamps and more particularly to an arrangement in which more than one lamp comprises a lighting unit. one of the lamps being an electric discharge lamp.

As known electric discharge lamps of the hot cathode high (vapour) pressure type have a much higher efiiciency than the gas filled tungsten filament lamp assuming that a normal life is allowed in each case.

In common with other discharge lamps the hot cathode lamp requires, to maintain'stable working, to have in series a current limiting device such as a choke in the case of an A. C. supply or a resistance in the case of a D. C. supply. It may be noted also that the use of a choke has an effect on the power factor and condensers are generally associated with the choke to bring about a correction of the power factor.-

The cost of this additional apparatus is a disadvantage and also militates against the use of discharge lamps for ordinary households and in places where wiring is already installed.

The present invention amongst other things has in View an arrangement which obviates the presence of such additional apparatus which is not in itself a source of illumination and. in fact is energy consuming.

In the present invention we arrange in series with a discharge lamp a lamp having a light emitting part which has a positive resistance-tem- "perature cpefiicient such as a tungsten filament lamp:

However, regard must be paid to the fact that such a" lamp will govern the rate of increase of light emitted by the discharge lamp in starting up and must withstand practically the whole of the mains voltage at the start and when the discharge lamp is in full working will have only a part of the mains voltage across it.

We arrange that the auxiliary lamp is of such a wattage that on starting it allows the discharge lamp to run up under the most favourable conditions and at a suitable time during the running up, part of the filament of the auxiliary lamp is put outpf action by being short circuited. Clearly also it would be possible to use morethan one auxiliary lamp and cut one or more of them out of circuit.

The cutting out of part of the filament of the auxiliary lamp may be done by an electromagnetic cut-out which is controlled by the voltage across the discharge lamp terminals but we prefer to use a thermal cut-out such as one of the lei-metallic type which is subjected to heat emitted by the discharge. lamp,

Electric discharge lamps of the hot cathode high pressure mercury vapour type are at present constructed with an inner. container in which are the electrodes and mercury filling and an outer container. The space between the inner and outer containers is exhausted and. incidentally the space is partly occupied by connecting and supporting wires. According to a further feature of the invention the auxiliary lamp .is situated in the outer container and the outer container may be enlarged to accommodate the filament of the auxiliary lamp. The enlargement may be at one end of the container which may be more convenient for mounting the auxiliary filament. On the other hand to get a better mixture of light the filament can be distributed adjacent to the surface of the inner container.

The space between the outer and inner containers may be filled with gases such as those used in the tungsten filament gas filled lamps. Also a thermal cut on such as a bi-metallic strip may be situated in the space and serve to short circuitaj portion of the filament of the auxiliary lamp.

It may be noted that the tungsten filament lamp contributes rays which compensate for the lack of 'red rays in the spectrum of a mercury filled gas discharge lamp although the latter may further useful purpose as will be made clear by the following.

In the hot cathode high pressure mercury vapour lamps the quantity of mercury contained in the lamp is so adjusted that when the lamp is in full working and at its designed wattage the mercury has been totally evaporated since the steepness of the volt-ampere curve in the case where thereis an-excess of mercury is a cause of unstable working. When evaporation is complete the current flowing may be increased without the voltage drop in the lamp increasing. This state of afiairs is however disadvantageous to the life of a tungsten filament lamp which is in series since for a given percentage increase of mains voltage, the percentage increase of volts drop'in the filament lamp is much greater. The volt-ampere curve of a similar lamp which has however additional metals such as cadmium and/or zinc in the filling is much less steep up to thepoint where complete evaporation of the.

it is arranged that in a series arrangement of a hot cathode high pressure electric discharge lamp the filling of which includes mercury and a lamp having a positive-resistance temperature coefiicient suflicient additional metals are included in the filling of the discharge lamp to ensure that with ordinary rises in the supply mains voltage the filling is not completely evaporated.

We may furthermore adjust the weights of cadmium and/or zinc relatively to the weight of the mercury in the filling so that as far as may be the voltage drop across the other lamp varies in the sameratio as the mains voltage varies. The effect of the introduction of additional metals in the lamp filling will be clear from a consideration of the curves shown in Figs. 1 and 2 of the accompanying drawings.

Fig. 1 shows current-voltage characteristic curves for two kinds of high pressure discharge lamps. Curve A is for a mercury vapour lamp without additional metals in the filling in which all the mercury is vapourized when the lamp is running. A steep rise will be noted commencing soon after the point at which the lamp strikes, and when all the mercury is volatiiized the maximum voltage across the lamp is reached and the curve becomes substantially horizontal showing that no appreciable change of voltage across the discharge path takes place with increase of current while the mercury is in a completely vapourized condition. If there is any increase in current fiow there must be an increase in voltage across the resistance connected in series with the discharge path and consequent increase in the watts to be dissipated, and the percentage rise in voltage across the filament will be greater than the percentage increase in applied voltage causing the current increase, the increase in voltage appearing almost entirely across the filament.

Curve B is a current-voltage characteristic for a high pressure mercury vapour discharge lamp with an additional filling of metals, e. g. cadmium and zinc, having a higher boiling point than mercury.

It will be noticed that during the running up of the lamp the curve rises much less steeply than does curve A and so longas there is unvapourized metal in the lamp the curve will tend to rise. If the amount of additional metal in the filling is such that it is not completely vapourized at a wattage in excess of the operating wattage that is likely to be encountered the lamp will then be operating on a sloping part of the curve and any increase in current will be accompanied by an increase in voltage across the discharge path. If

v the filling is such as to be completely vapourized at the operating wattage the curve will assume the fiat top characteristic of curve A. By a suitable adjustment of the relative weights of the metal contents a characteristic curve may be had closely approximating that of a tungsten filament lamp.

In Fig. 2 are two sets of curves showing how the voltage varies across the discharge path and across an incandescent tungsten filament connected in series with it with changes in applied voltage, the voltages being expressed as percent ages. Curve C shows the change of voltage across the discharge path of a mercury vapour lamp and it will be, found to have a fiat top overthe working range in accordance with the characteristic of curve A in Fig. 1. Any rise in applied voltage produces a change of current through the lamp and since no appreciable change across the discharge path occurs, the increased voltage appears across the incandescent filament as is shown by the steeply rising curve D. This shows that for a 5% increase or decrease in applied voltage a change of approximately 10% appears at the ends of the filament. It will accordingly be realized that the life of the filament is jeopardized. If, in addition to mercury additional metals such as cadmium and/or zinc are included in the filling and the amount of metal is such that it is not completely vapourized at the operating wattage or any excess thereof which is likely to occur in practice the characteristic becomes changed in that it is no longer fiat topped for the discharge path but rising as shown by curve E. Due to increase in voltage across the discharge path with increase in current there will not be the great increase in voltage across the filament that is shown by curve D but the characteristic for the filament will take the form shown at F. Any change in applied voltage .will then appear across the discharge path and across the filament in a ratio determined by the slope of curves E and F. If curve E be made to correspond closely with curve F a given percentage change in applied voltage will produce a like percentage change across the filament.

A lighting unit comprising a high pressure va pour discharge lamp and a tungsten filament lamp according to the invention will now be described with reference to Figs. 3, 4 and 5 of the accom panying drawings. Figs. 3 and 4 show two views of such a lamp, the view had in Fig. 4 being that obtained by looking at Fig. 3 from the right and Fig. 5 being a plan view of the filament projected from Fig. 4.

The lamps are contained within a glass outer envelope l fitted with a screw cap of known type.

emissive material closely surrounded by a helix' of tungsten or tantalum wire having its axis at right angles to that of the discharge lamp, an auxiliary tungsten. electrode of helical form coa-xial with that of the lamp surrounding the active electrode. The wires of the electrodes are joined to light rods which are sealed into the ends of the discharge lamp envelope and the connections to the wires of the upper electrodes pass from the seal into the outer envelope l where they are sealed into the pinch 5 at the capped end of envelope I. Also sealed into the pinch 5 are rods 6 and 1. Rod 6 is connected to a helical ring 0 fitting the inside of the envelope I and surrounding the discharge lamp near its lower end. The ring 8 has a stirrup portion 9 depending below the discharge lamp 2 to which are attached the rods sealed into the lower end of the discharge lamp and connected to the lower active electrode.

Rod 1 is suitably bent or has a branch "la'con-.

nected to it at the upper end of the outer envelope, the part 1a passing down the envelope to apoint below the discharge lamp where it supand a support and which is attached to the stirmp 9. A further point in the filament is connected by rod III which passesup the outer envelope and terminates in a glass bead ll. Near the glass bead it is provided with a contact memher I! associated with the thermal switch 4. Rod 1 or a branch thereof is also sealed into the glass bead H and provided with a contact member attached to the bi-metallic member of the thermal switch 4. The tungsten filament 3 is supported by radial members sealed into a boss I3 which is centred by supporting members H attached to the stirrup 9 and the rods 6 and Ill. The connections to the upper electrode of the discharge lamp by which the lamp itself is supported are held by a spiral supporting spring l5 which by its tension fits the inside of the envelope l closely and provides a resilient suspension for the discharge lamp 2. The rod I and a wire connected to the active electrode lead l5 are attached to the cap terminals, i. e. to the screwed portion and to a central terminal. Wires connected to the auxiliary electrode lead l1 and to rod 6 are connected to a high resistance contained within the lamp cap. The sealing pip for the discharge lamp is made as small as is practicable and is positioned at the upper end behind the electrode where it may be kept hot by convection currents. Both hemi-spherical ends of the discharge lamp are preferably coated internally with a thin metallic coating to assist in the volatilization of the metal filling by reflection.

The space between the discharge lamp 2 and the outer envelope l is exhausted and an inert gas or gases are introduced so that the filament lies in a gas filled envelope and has the properties of the well known gas filled incandescent filament lamp.

The filament contributes red rays which will, at least to some extent compensate for the lack of red rays in the spectrum of a mercury filled lamp. Further compensation may be had by the introduction of other metals such as cadmium and/or zinc into the discharge lamp and these metals play a further important part in that the characteristic of the discharge lamp is modified so that the current volts characteristic, by suitable adjustment of the weights of the metals can be made to approximate to that of a gas filled tungsten filament lamp with the result. that any change in the voltage applied to the lighting unit appears equally across the discharge path and across the filament.

To avoid overloading the filament during the running up period of the discharge lamp and yet permit the filament to glow at its rated output it is arranged that the whole filament is connected in series with the discharge lamp during the running up of the latter, the length of filament corresponding to the difference between the applied voltage and the voltage drop across the discharge path.

The bi-metallic thermal switch is situated near the top of the discharge lamp so that it is affected almost solely by the heat dissipated by the discharge lamp, the heat from the filament having very little effect. After the discharge isstarted the bi-metallic member heats up by conduction and convection and the switch is so adjusted that it closes when the normal running up period of the lamp has expired or is about to expire and short circuits a length of the filament correspond ing to the voltage drop across the discharge path so that the remaining portion of the filament glows at its rated output. Owing to a certain amount of thermal lag it is assured that the temperature of the bi-metallic member is still increasing after the voltage across the discharge path has become constant.

As an example of suitable data for the production of a lamp embodying the invention a lamp rated at 500 watts will be considered. The power factor of such a lamp will be about .94 so that with 534 volt-amperes and 230 applied volts the full load current will be 2.32 amperes. The voltage drop across the discharge path immediately after the-lamp has been switched on is about 20 so that the total length of filament will correspond to 210 volts. The voltage across the discharge lamp will rise to about 138 and the length of filament to be short circuited by the thermal switch will correspond to this value. The portion of filament left in circuit will then be operated at its rated efficiency of 14.5 lurnens per watt calculated to give a life greater than 1500 hours. There will thus be a watts dissipation of approximately 200 in the filament and 300 in the discharge lamp.

As regards the metallic filling of the discharge lamp the amount of additional metals will not require to be so great as in the case of the lamp described in United States application No. 755,989 since the filament itself compensates, at least partially for the lack of red in the mercury spectrum and the additional metals are mainly introduced to bring about the desired slope of the current volts characteristic curve. For this purpose an amalgam consisting of 1 part by weight of cadmium, 1 of zinc and 10 of mercury amounting to a total weight of .35 gramme, will be found satisfactory. The lamp 2 may have diameter of 38 mm. and a length of discharge of l40'millimetres. The resistance in the starting circuit situated in the lamp cap may be 50,000 ohms.

What is claimed is:-- r

1. An electric lighting unit comprising a high vapour pressure discharge lamp in which the filling consists of mercury and an additional metal having a higher boiling point than mercury, a tungsten filament connected in series with said discharge lamp and situated underneath said discharge lamp, a containing vessel enclosing said discharge lamp and said tungsten filament and filled with an inert gas at low pressure, and a thermally operated device situated in said containing vessel for cutting out a portion of the said tungstenrfilament when the unit is connected to supply mains and. the voltage across the discharge lamp approaches closely its working value,

the amount of additional metal in the filling of r the discharge lamp being such as to give the discharge lamp when in full working a currentvoltage characteristic approximating to that of a gas-filled tungsten filament lamp.

2. An electric lighting unit comprising a high pressure discharge lamp having a filling consisting of an amalgam of mercury, cadmium and zinc, a metal filament connected in series with said discharge lamp, and a thermally operated device for cutting out a portion of the metal filament when the unit is connected to supply mains and the voltage across the discharge lamp approaches closely its working value, the proportions of the metals in the filling of the discharge lamp and their amount being such as to give the discharge lamp when in full working a current-voltage characteristic approximating to that of the metal filament connected in series with it.

3. An electric lighting unit comprising a high vapour pressure discharge lamp having a filling of mercury and additional metal with a boiling point higher than mercury, a tungsten filament connected in series with said discharge lamp, a containing vessel enclosing said discharge lamp and tungsten filament, and a thermally operated 4. In an electriclighting unit as set forth in claim 3, the additional metal being cadmium and zinc. v

5. In an electric lighting unit as set forth in claim 3, the additional metal being cadmium and zinc and the proportions of mercury, cadmium and zinc being approximately in the order of 10 to 1 to 1; a

WILLIAM HENRY LE MARE'CHAL. JOHN NORMAN ALDINGTON. 

